Vibration isolation is often used to stabilize a payload's angular orientation in the face of translational vibration. The translational vibration may be self induced by the payload (for example by self induced vibration from a motor of a gimbal assembly, or transmitted from a support structure upon which the payload is supported. Rotational motion of a payload may be reduced (or nearly eliminated) if the payload is supported by a suspension system that does not couple linear base motion vibration into angular motion. Such a suspension system may be referred to as a vibration isolator system.
Conventionally, such coupling may be prevented by providing elastic mounts (or struts) having attachment points to the payload in a common plane containing the payload's center of mass (“cm”). This places the “elastic center” of the isolators at the cm of the payload. The meaning of the term “elastic center”, as used herein, may be better understood by considering a hypothetical suspension system including a plurality of elastic struts supporting an object, for example, a payload. The elastic center of the suspension system is the point at which, if the center of mass of the body is located at the point, the application of a force through the point would result in a pure translational movement, and the application of a moment about the point would result in pure rotation of the body about that point.
A problem exists for many system configurations where it is impossible to provide elastic struts having attachment points to the payload in a common plane containing the payload's cm. In optical systems, for example, the area of the cm plane may need to remain clear for the optical field of view for the optical system. For such systems, the elastic struts must be coupled to an attachment surface of the payload assembly or system that is substantially distant from the payload's cm.
Attachment points at which the elastic struts connect to the payload may collectively define a mount plane. As the term is used herein, a “projected elastic center” of elastic struts in a suspension system may be understood by recognizing that each elastic strut has a respective line of action (defined by its longitudinal axis) at an angle of orientation with respect to the mount plane. If the lines of action are each oriented at 90 degrees to the mount plane, so that the lines are parallel and do not intersect, the elastic center of the suspension system will be in the mount plane (i.e., the elastic center is not projected). Contrariwise, if the lines of action are not oriented at 90 degrees to the mount plane a projected elastic center will generally exist at some distance from the mount plane.
A special case where the projected elastic center coincides with a center of mass of the supported body, occurs when the line of action of each elastic mount (or “strut”) passes through the center of mass of the supported body. As illustrated in FIG. 1, for example, a payload 110 is supported from a base structure 102 by elastic struts 103, each strut 103 having a line of action passing through the center of mass 120 of payload 110.
As disclosed in Denice, Jr., et al., U.S. Pat. No. 6,871,561 (hereinafter, “Denice”), when lines of action of elastic mounts (or isolators) intersect at the center of mass of a supported body, cross-coupling of translational vibration into rotational motion can be substantially eliminated.
Gran, et al., U.S. Pat. No. 6,022,005 (hereinafter, “Gran”) discloses another arrangement for preventing the coupling of translational vibration unwanted rotational movements. According to Gran, three pairs of semi-active isolators are provided. The isolators in each pair are positioned in a parallel relationship with each other, lying in the same plane such that a centerline parallel to and midway between the two isolators of each pair passes through the center of mass of the payload.
The geometric relationships prescribed by the schemes disclosed in Gran and Denice are difficult or impossible to achieve in many real world situations. Referring now to FIG. 2, for example, where a payload attachment structure 201 is attached to a base attachment structure 202 by several elastic struts 203, available mounting places on the respective attachment structures do not permit the struts 203 to be arranged so that their line of action is directed toward center of mass 220 of the payload (not shown).